Digital transmission of encoded moving-image television signals having a frequency bandwidth of 4 MHz generally requires a transmission rate of about 100 Mbit/s. Since a primary bit rate in a multiplex digital transmission network is of about 1.5 Mbit/s, for example, it is necessary to encode the above television signals highly efficiently in order to transmit the encoded television signals in the multiplex digital transmission network.
A digital image communication apparatus according to the present invention effects interframe encoding for encoding only image changes between frames and transmitting the encoded image information.
FIGS. 1(A) and 1(B) of the accompanying drawings are schematic block diagrams of an overall arrangement of a conventional digital image communication apparatus. FIG. 1(A) shows a transmitter of the conventional digital image communication apparatus, and FIG. 1(B) shows a receiver of the conventional digital image communication apparatus. In the transmitter, a preprocessor 171 converts an input image signal into a digital image signal and filters the image signal, and a next source encoder 172 suppresses the redundancy of the image. A variable-length encoder 173 assigns a code of a short bit length, i.e., a so-called variable-length code, to data which are generated highly frequently. Finally, a multiplexer 174 multiplexes the variable-length code and data such as audio, and outputs the multiplexed data to a network.
The receiver effects a process which a reversal of the process carried out by the transmitter, using a demultiplexer 175, a variable-length decoder 176, a source decoder 177, and a postprocessor 178, and outputs a reproduced image.
The source encoder 172 effects an interframe encoding process for storing a preceding frame (frame), comparing a new present frame with the preceding frame, and encoding only an area where the image has changed, in order to reduce the number of transmitted bits with respect to an area where no image motion occurs in the frame. In this interframe encoding process, since the receiver is supplied with only information representing the difference (predictive error) between the preceding frame and the present frame, if a data error or a data slip (cell loss) occurs even once, then the reproduced image in the receiver is and remains degraded continuously. A leaky prediction process is employed to prevent such a drawback. The leaky prediction process is a process of determining a predictive error after the data of the preceding image is multiplied by a leak coefficient .alpha. (0&lt;a&lt;1). When the leaky prediction process is carried out, the effect of a deterioration of the reproduced image in the past converges to 0 with time for automatic recovery from such a deterioration of the reproduced image.
The internal structures of the source encoder 172 and the source decoder 177 will be described in detail below with reference to FIGS. 2(A) and 2(B) of the accompanying drawings. FIG. 2(A) shows the source encoder 172 and FIG. 2(B) shows the source decoder 177. The source encoder 172 effects a motion compensation leaky prediction process.
In the source encoder 172, a motion vector detector 181 checks each of a plurality of areas of the preceding frame to search for an area which is closest to each of the areas of the present frame, based on the present frame data from the preprocessor 171 and the preceding frame data, and detects the difference between the closest areas of the preceding and present frames as a motion vector. The motion vector is outputted to the variable-length encoder 173 and a memory 182. The memory 182 stores the data of each area of the preceding frame. The data of the area of the preceding image which is associated with the area of the present image that is indicated by the supplied motion vector are read from the memory 182 and delivered to a multiplier 183. The multiplier 183 multiplies the supplied data by a leak coefficient a (0&lt;a&lt;1), and outputs the product data to a subtractor 184. The subtractor 184 subtracts the output data from the multiplier 183 from the area data of the present frame from the preprocessor 171, producing a predictive error. The predictive error is quantized in predetermined steps by a quantizer 185, and the quantized predictive error is outputted to the variable-length encoder 173.
The quantized predictive error is converted back, or dequantized, to the original predictive error by a dequantizer 186, and the output data from the multiplier 183 are added to the original predictive error by an adder 187. By this addition, the present frame data supplied from the preprocessor 171 are restored, and then newly stored as preceding frame data in the memory 182 in preparation for a next cycle of processing. The memory 182 now stores the preceding frame data which contain a quantization error that are identical to the preceding frame data stored in the receiver.
In the source decoder 177 in the receiver, when a motion vector is supplied from the variable-length decoder 176, the data of the area of the preceding image which is associated with the area of the present image that is indicated by the supplied motion vector are read from a memory 191, and the read data are multiplied by the leak coefficient .alpha. by a multiplier 192. A quantized predictive error outputted from the variable-length decoder 176 is dequantized back to its original predictive error by a dequantizer 193. Then, the original predictive error is added to the frame data as multiplied by the leak coefficient .alpha. by an adder 194. This addition produces decoded data of the present frame, which are outputted to the postprocessor 178. The memory 191 now newly stores the decoded data of the present frame as preceding frame data in preparation for a next cycle of processing.
Images transmitted by the digital image communication apparatus which effects the above predictive encoding process are supposed to experience limited image motions in the frames. For such images, the encoding efficiency is increased based on the fact that the predictive errors between the data of preceding and present frames essentially concentrate nearly on 0.
With the leaky prediction, however, there occurs a phenomenon in which the predictive errors do not necessarily concentrate nearly on 0. Such a phenomenon will be described below with reference to FIGS. 3(A) and 3(B). FIGS. 3(A) and 3(B) are graphs showing predictive errors that are produced by the multiplication by the leak coefficient .alpha. when the data of the preceding frame and the data of the present frame are equal to each other (there is no motion of images).
It is assumed that an analog input image signal is converted to an 8-bit digital image, i.e., a digital image having 256 levels, i.e., a range (numerical ranges) of 0.about.255, for example, by the preprocessor 171 shown in FIG. 1(A).
FIG. 3(A) shows a predictive error that is produced by the subtractor 184 shown in FIG. 2(A) in such a case. Since the data of the preceding frame and the data of the present frame are equal to each other, the predictive error should be 0 irrespective of the data level in the absence of the multiplication by the leak coefficient .alpha.. However, because the multiplication by the leak coefficient .alpha. is carried out, the predictive error does not become 0 at the other data levels than the data level 0.
The predictive error can be expressed by the following equation: EQU Predictive error=the data level of the present frame-the data level of the preceding frame.times..alpha..
FIG. 3(B) shows a predictive error produced when an analog input image signal is converted to a digital image signal with the range corrected into a range -128.about.127 (in which the number of the range level 256 is not varied).
In each of FIGS. 3(A) and 3(B), the predictive error becomes large as the level of the data (preceding frame data) multiplied by the leak coefficient departs from 0. Stated otherwise, those images which are composed of many data whose data levels depart from 0 have a problem in that the encoding efficiency is greatly lowered by the leaky prediction.